Modelskill

Re-work on how observations and modelresults are connected. Observations are no longer "added" to the modelresult. With this release, a new object "Connector" has been introduced. It is responsible for connecting observations and modelresults, including validation and extraction of modelresult data at observation positions.

Note! This release has breaking changes!

* Taylor diagram
* Quick and dirty compare by fmskill.compare()
* Bugfixes

Skill object with better visualization possibilities for skill assessments. The main methods of the new class are:
* style()
* plot_bar()
* plot_line()
* plot_grid()
* sel()

spatial skill class
command-line interface
more metrics
test for overlapping data
infer model item
temporal coverage plot
more validation
fix non-unique time index

spatial skill assessment
multi-variable assessments

score() added to comparer
bugfixes